Many steering systems provide variable ratio steering. Such variable ratio steering systems have been disclosed in Japanese Patent Provisional Publication No. 4-159178 (hereinafter is referred to as “JP4-159178” and corresponding to U.S. Pat. No. 5,265,019) and Japanese Patent Provisional Publication No. 7-257406 (hereinafter is referred to as “JP7-257406” and corresponding to U.S. Pat. No. 5,489,004).
The variable ratio steering system disclosed in JP4-159178 includes a transmission ratio changing mechanism arranged between a steering wheel and tie-rod linkages, an actuator driving the transmission ratio changing mechanism, steering sensor means, a vehicle speed sensor, and a controller. The transmission ratio changing mechanism serves to change a transmission ratio of a steer angle at steerable front wheels with respect to a steering wheel rotation angle, simply a steering wheel angle. In order to monitor a turning state of the steering wheel, the steering sensor means is comprised of a turning angle sensor, a turning angular velocity sensor (an angular turning speed sensor), and a transverse acceleration sensor, respectively detecting the steering wheel angle (a turning angle), the turning angular velocity of the steering wheel (i.e., the time rate of change in the steering wheel angle), and the lateral acceleration acting on the vehicle body. The controller controls the actuator on the basis of the sensor signals from the vehicle speed sensor and the steering sensor means. The controller generates a control signal to the actuator so that the steer angle at the front wheels is increasingly compensated for, when (i) a first condition wherein the vehicle speed is greater than or equal to a predetermined speed that the front wheels oscillatingly returns to the straight-ahead position after a turn is completed and the steering wheel is released, and (ii) a second condition wherein the turning angular velocity of the steering wheel is greater than or equal to a reference value, are both satisfied.
The variable ratio steering system disclosed in JP7-257406 includes an input shaft to which the steering wheel torque is delivered, an output shaft that transmits a steering force (or a steering torque) acting on the steered road wheel for steering, a first shaft (for example, the input shaft itself) mechanically linked to one of the input and output shafts, a second shaft (for example, the output shaft itself) mechanically linked to the first shaft such that relative movement of the second shaft to the first shaft is permitted, actuating means that causes the relative movement of the second shaft to the first shaft, and a third shaft integrally formed with or integrally connected to the second shaft such that the axis of the third shaft is eccentric to the axis of the second shaft. The drive means is comprised of a support member rotatably supported by a casing in such a manner as to rotatably support the first or second shafts at an eccentric position to the center of rotation of the support member, and a drive source that causes rotary motion of the support member. With the aforesaid arrangement of the variable ratio steering system of JP7-257406, it is possible to reconcile two requirements, that is, good handling (in other words, production of a greater road-wheel steer angle by a small steering wheel input) during low-speed driving, and stable vehicle behavior (in other words, prevention of an excessively high steering response) during high-speed driving.